Indexing text files using transient state in Clojure

gistfile1.clj

;; Create in-memory mappins from each word to a set of files that contain it.
;; If you kill the server, you'll have to reindex everything.
(defn build-index[dirname]
  (loop [files (map #(.getName %) (.listFiles (java.io.File. dirname))) idx (transient {})]
    (if (empty? files) (persistent! idx)
        (recur
         (rest files)
         (loop [tokens (-> (str dirname "/" (first files)) slurp tokenize)
                iidx idx]
           (if (empty? tokens) iidx
               (recur
                (rest tokens)
                (assoc! iidx (first tokens) (union (iidx (first tokens)) #{(first files)})))))))))